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What colour is the auxiliary line on a three-line braking system?

Blue
Correct answer

DVSA explanation

If you're driving an articulated vehicle or a trailer combination, it's vital that you understand the rules that apply to coupling and uncoupling the brake lines. If you take a practical driving test with a trailer, you'll be expected to demonstrate this during the test. The lines are colour-coded: red is the emergency line, blue is the auxiliary line and yellow is the service line. They must be connected strictly following the correct procedure.

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